1. Field of Invention
This invention relates to apparatus and a method of sensing and monitoring strain in concrete structures. More specifically, it relates to a helical optical fiber sensor embedded in concrete, and a method of using the sensor, to monitor the deformation (strain) of concrete due to compression, tension, and bending moment.
2. Description of Prior Art
Strain in concrete structures, such as buildings, foundations, bridges, and dams may cause the failure of such structures, usually with little or no warning. Early detection of strain in concrete structures provides a means of reinforcing or repairing strained regions of concrete before failure occurs. Visual inspection has been found inadequate for monitoring strain in concrete structures. Strain gauges have been used in the past for monitoring strain in concrete structures. They are bonded to the structure, and their electrical resistance changes as strain (deformation) occurs. Optical fiber strain sensors as here disclosed have a higher dynamic range and higher reliability than strain gauges.
Fiber optic sensing means have been disclosed in the prior art. U.S. Pat. No. 5,381,492 discloses a fiber optic vibration sensor having two single-mode optical fibers, one providing a reference signal, and the other one subject to vibrational displacement providing a signal indicative of the perturbation of the sensor. U.S. Pat. No. 5,381,005 describes an optical fiber stress detector, for detecting stresses in structures, using an amplitude-modulated monochromatic light signal that is directed into a reference fiber and a sensing fiber, the phase shift of the sensing fiber output with respect to the reference fiber output being an indication of the stress applied to the detector fiber. This device does not measure strain directly. U.S. Pat. No. 4,686,631 provides an improved method of determining internal stress in polycrystalline solids by X-ray diffraction. U.S. Pat. No. 5,563,348 discloses a stress sensor using changes in the natural frequency of an oscillating string as a means of measuring stress.